1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell stack, a reforming unit, a raw fuel supply unit, and a water supply unit. Further, the present invention relates to a method of operating the fuel cell system.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, a predetermined number of the MEAs and the separators are stacked together to form a fuel cell stack.
As the fuel gas supplied to the fuel cell, normally, a hydrogen gas generated from hydrocarbon raw material by a reformer is used. In general, in the reformer, a reformed raw material gas is obtained from hydrocarbon raw material of a fossil fuel or the like, such as methane or LNG, and the reformed raw material gas undergoes steam reforming, partial oxidation reforming, or autothermal reforming to produce a reformed gas (fuel gas).
For example, in a fuel gas supply apparatus used for a power generation apparatus including a solid polymer electrolyte fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2006-260874, as shown in FIG. 8, a fuel gas supply line 2 is provided downstream of a fuel pump 1. The fuel gas supply line 2 is connected to the fuel pump 1, and a reformer 4 of a fuel processing apparatus 3 is connected to the fuel gas supply line 2. A desulfurizer 5 and a flow rate meter 6 are provided in the fuel gas supply line 2. The flow rate meter 6 is provided downstream of the desulfurizer 5.
The desulfurizer 5 functions as a pressure regulator. After the flow of the city gas supplied to the desulfurizer 5 is rectified, the city gas is discharged from the desulfurizer 5. According to the disclosure, in the structure, the pressure variation and the flow rate variation due to pulsation of the city gas supplied from the fuel pump 1 are suppressed by the desulfurizer 5.
However, in such a conventional technique, for example, when it becomes necessary that the fuel cell needs to be operated for a high load suddenly, and a large amount of fuel is requested to the fuel processing apparatus 3, by the pressure regulating operation of the desulfurizer 5, the load on the pump 1 becomes heavy. Under the circumstances, it is not possible to rapidly supply the fuel to the reformer 4. In view of the above, the size of the fuel pump 1 itself may be increased. However, the large fuel pump 1 cannot be used in practical applications.
Further, at the time of stopping operation of the fuel cell, since the desulfurizer 5 has the pressure regulating function, the fuel gas supply line 2 has a residual pressure of the fuel gas. Due to the residual pressure, the unnecessary fuel gas is supplied to the reformer 4. Therefore, operation of the fuel cell cannot be stopped promptly.